1. Field of the Invention
This invention relates to a particle beam processing apparatus and materials treatable using such apparatus. In particular, this invention relates to a particle beam processing apparatus including a particle generating assembly, a foil support assembly having a thin foil, and a processing zone. When the treatable material is exposed to the particles generated by the apparatus, the particles cause a chemical reaction on a coating of the material.
2. Description of the Related Art
A particle beam processing device is commonly used to expose a substrate or coating to highly accelerated particle beams, such as an electron beam (EB), to cause a chemical reaction on the substrate or coating.
An electron is a negatively charged particle found in all matter. Electrons revolve around the nucleus of an atom much like planets revolve around the sun. By sharing electrons, two or more atoms bind together to form molecules. In EB processing, energetic electrons are used to modify the molecular structure of a wide variety of products and materials. For example, electrons can be used to alter specially designed liquid coatings, inks and adhesives. During EB processing, electrons break bonds and form charged particles and free radicals. These radicals then combine to form large molecules. By this process, the liquid is transformed into a solid. This process is known as polymerization.
Liquid coatings treated with EB processing may include printing inks, varnishes, silicone release coatings, primer coatings, pressure sensitive adhesives, barrier layers and laminating adhesives. EB processing may also be used to alter and enhance the physical characteristics of solid materials such as paper, substrates and non-woven textile substrates, all specially designed to react to EB treatment.
A particle beam processing device generally includes three zones, i.e., a vacuum chamber zone where a particle beam is generated, a particle accelerator zone, and a processing zone. In the vacuum chamber, a tungsten filament is heated to about 2400K, which is the thermionic emission temperature of tungsten, to create a cloud of electrons. A positive voltage differential is then applied to the vacuum chamber to extract and simultaneously accelerate these electrons. Thereafter, the electrons pass through a thin foil and enter the processing zone. The thin foil functions as a barrier between the vacuum chamber and the processing zone. Accelerated electrons exit the vacuum chamber through the thin foil and enter the processing zone at atmospheric conditions.
Electron beam processing devices that are commercially available at the present time generally operate at a minimum voltage of 125 kVolts. These existing EB units utilize thin foil made of titanium having a thickness of 12.5 micrometers, to cure coatings on substrates that are being fed through the processing devices at a rate of 800–1000 feet per minute. Such an EB unit may be purchased from Energy Sciences, Inc. of Wilmington, Mass., Model No. 125/105/1200. The current technology cannot be used in certain industries, e.g., flexible food packaging. An EB unit operating at 125 kVolts deposits substantial amounts of the energy onto the polyethylene based sealant films which contact the food being packaged. This deposit causes off-odors in the films and increases its seal initiation temperatures.
The efficiency of such EB units can be improved by reducing the operating voltage below 125 kVolts. In addition, operating below 125 kVolts allows better control of the depth of energy deposition and minimizes the electron energy absorbed by the sealant films. However, when the voltage is reduced below 125 kVolts, the kinetic energy of the electrons traveling through the titanium foil decreases because more energy is being absorbed by the titanium foil, causing the foil to heat up excessively. Excessive heat causes the titanium foil to become blue, brittle, and lose its mechanical strength. Excessive heat also poses a problem with heat management of the system. Consequently, the feed rate of the substrate must be substantially reduced which makes the processing device commercially unviable.
In light of the foregoing, there is a need for a particle beam processing device that operates more efficiently, is smaller in size, has a reduced power demand, and is cheaper to construct.